The invention relates to a method of determining the intramucosal pH of an animal or human stomach or gastrointestinal tract, comprising the determination of the carbon dioxide tension in the arterial blood and the determination of a carbon dioxide tension in the blood flowing through the wall of the stomach or gastrointestinal tract, by introducing into the stomach preferably a tube with a balloon which is permeable only for carbon dioxide and which is filled with a saline solution, and after a previously determined retention time of at least 30 minutes, removing said saline solution for analysis of the carbon dioxide fraction present in the saline solution.
Such a method is known from published International Patent Application WO 94/21163. By the known method the pH of, for instance, a stomach wall is determined by the assumption that the partial carbon dioxide pressure in the surface layers of the stomach wall is in equilibrium with those of the deeper layers of the stomach wall. A further assumption is that the bicarbonate concentration in the tissue is the same as the bicarbonate concentration in the arterial blood. The pH in the tissue is then determined by applying a modified version of the known "Henderson-Hasselbalch" equation, which reads: EQU pHa=6.1+log[HCO.sub.3 ]-log[PaCO.sub.2.]
The terms pHa and PaCO.sub.2 in this formula relate to the arterial blood. For the present objective the formula is modified by replacing PaCO.sub.2 with PiCO.sub.2 that is to say the carbon dioxide pressure in the stomach wall, and to assume as mentioned above, that the concentration of the bicarbonate in the tissue is in equilibrium with the bicarbonate concentration in the arterial blood. However, the problem is that this latter assumption is not correct, particularly with patients who are critically ill and are in septic or anaphylactic shock. As a result the pH measured according to the known method deviates from the actual pH, thus hindering a correct, and timely treatment of the patient.
In the known method, measurement of the partial carbon dioxide pressure in the stomach is obtained by inserting a silicone balloon containing a saline solution into the gastrointestinal tract whereby the carbon dioxide pressure in the saline solution should equilibrate with the carbon dioxide pressure in the gastrointestinal tract. Then the time required to achieve equilibration is measured and the partial carbon dioxide pressure in the saline solution is determined using a blood gas analyzer while, if necessary, a nomogram is used for the determination of the stabilized carbon dioxide value, departing from the stabilization time and the carbon dioxide pressure measured in the saline solution in combination with the bicarbonate concentration in a substantially simultaneously taken arterial blood sample.
To speed up the pH determination of the organ to be examined, WO 94/21163 suggests to compare the carbon dioxide pressure relating to the respective organ with the arterial bicarbonate concentration and/or another direct or indirect measurement of a global or systematic physiologic value, such as the pH, the carbon dioxide pressure or oxygen pressure of arterial, venous or other kind of blood, mixed venous bicarbonate, the carbon dioxide pressure in the exhalation air or other values and, in order to determine whether the condition of the respective organ renders it desirable that a) a bicarbonate concentration should be determined and/or b) what kind of clinical therapy or interference is necessary or desirable with regard to the oxygen supply to the respective organ.